1. Field of the Invention
The present invention relates to a monolithic magnetic apparatus and the process for making such apparatus.
The present invention relates to a monolithic magnetic apparatus and the process for making such apparatus as defined in the preamble of claim 1.
As used herein, the term magnetic apparatus is intended to indicate:                a permanent-magnet apparatus, i.e. an apparatus that does not require any power supply when used for clamping or for changing its state from active to inactive and vice versa, and is formed with permanent magnets in appropriate arrangement within the apparatus;        an electro-permanent apparatus, i.e. an apparatus that does not require any power supply when used for clamping and requires power supply when it is activated and inactivated, and is formed with reversible permanent magnets and, if needed, with static permanent magnets in appropriate arrangement within the apparatus;        an electromagnetic apparatus, i.e. an apparatus that requires power supply when used for clamping, whose magnetic core is made of ferromagnetic material.        
2. Description of the Related Art
In prior art, also with reference to FIGS. 1A and 1B, the process for making a magnetic clamping apparatus 1 for example of the electro-permanent dual-magnet type, includes a first step in which a housing 2 is made from a block, to form a bottom 2B, with number “N” of pole pieces 3A arranged on its inner surface 2C.
Otherwise, the housing 2 may be formed by assembling together various components with methods well known to those skilled in the art.
Each pole piece 3A (see FIG. 1B), in the case of an electro-permanent dual magnet apparatus, comprises at least:                one pole piece collector 5,        one reversible permanent magnetic core 4 and        one electric coil 3 (also known as solenoid) for changing the magnetization state of the reversible permanent magnetic core 4, which extends around the reversible permanent magnetic core 4.        
Particularly, it can be seen from such FIG. 1B that the electric coils 3 have such a configuration as to define a space for receiving the reversible permanent magnet 4, such as a magnet of the AlNiCo type, above which the pole piece collector 5 is placed.
Furthermore, the pole pieces 3A may also comprise one or more static magnets 9, e.g. made of ferrite or NdFeB, also suitably oriented, that can generate an additional permanent magnetic field for clamping the ferrous element P.
Still with reference to FIGS. 1A and 1B, it can be noted that the pole piece collector 5 of each pole piece 3A consists of a substantially parallepepipedal ferromagnetic piece having a square plan shape.
It shall be noted that, as used herein, the term pole piece collector 5 is intended to indicate an element that has one side 5A whose surface is magnetically neutral when the magnetic apparatus 1 is inactivated and magnetically active when the magnetic apparatus 1 is activated.
Thus, in other words, the pole piece collector 5 may have four of its six surfaces in which the magnetic field is oriented in one direction, the fifth surface in which the direction of the magnetic field can be changed into a polarity that is identical or opposite to the magnetic field in the other four surfaces and a sixth surface 5A, which is:                magnetically neutral when the magnetic field generated on the fifth surface has a polarity opposite to the magnetic field of the other four surfaces (the magnetic apparatus is inactive) or        magnetically active when the magnetic field generated on the fifth surface has the same polarity as the magnetic field of the other four surfaces (the magnetic apparatus is activated).        
In short, the pole piece collector 5 is an element designed to convey the magnetic flux generated by the reversible permanent magnetic core 4 to the surface 5A to form the magnetic clamping surface 2A.
It shall be noted that the surfaces 5A of the “N” pole piece collectors 5 form together the magnetic clamping surface 2A for firmly clamping the ferrous elements P to be machined and/or for other operations.
In other words, when the apparatus 1 is in the active state, the surfaces 5A of the “N” pole piece collectors 5 are activated to magnetically clamp the ferrous elements P.
It should be further noted that the term activate/inactivate, concerning the magnetic apparatus, is intended to indicate the possibility of changing the magnetization state of the reversible permanent magnet 4 by the action of a suitable electromagnetic field generated by the electric coil 3.
Then, the process includes the step of associating the pole pieces 3A with the housing 2, for example, by means of a screw 6 received in a suitable hole 7, so that the solenoid 3-reversible magnet 4 assembly can be clamped into a pack.
Particularly, each pole piece collector 5, which is known to be a separate element, is accommodated within the housing 2 and fastened to its bottom 2B by the screw 6.
The screw 6 is usually placed in a central position relative to the pole piece collector 5.
Nevertheless, by simply fastening the pole piece collector 5 to the bottom 2B of the housing 2, there is no guarantee that the pole piece collector will maintain its stability during machining of the clamped workpiece P.
Indeed, the pole piece collectors 5 of each pole piece 3A are very sensitive to vibrations produced during machining of the clamped workpiece P, which cause detrimental effects in terms of accuracy and life of the magnetic clamping apparatus.
Furthermore, pole extensions 14 may be respectively associated with one or more pole pieces 3A, when the use of such extensions is specifically needed for machining the ferrous elements P.
The use of pole extensions 14 increases the mechanical stresses induced on the respective pole piece collectors 5, because they somewhat act as lever arms and further enhance vibrations.
Furthermore, a filling step 10 is also provided, whereby the magnetic apparatus 1 can be made substantially impervious to impurities and/or liquid infiltrations, and any gaps can be filled.
An exemplary material to be used for filling is resin 100.
The clamping surface 2A so obtained is cohesive but is formed using several different types of materials.
Thus, the clamping surface 2A is made from both the ferromagnetic material that forms each pole piece collector 5 and the filling material that surrounds such pole piece collector 5.
This results in a different sturdiness of the clamping surface 2A, mainly due to the filling material (such as resin); resin is less resistant and affects the overall sturdiness of the clamping surface 2A and sets the limit of the stresses that can be exerted on the workpiece P to be machined.
Furthermore, the difference among the thermal expansion coefficients of the materials that form the clamping surface 2A affects the quality and accuracy of machining of the ferrous workpieces P as the operating temperature of the magnetic apparatus 1 and the temperature of the clamping surface 2A change.
However, the bottom 2B of the housing 2 of the magnetic apparatus 1 is formed from one type of material, e.g. ferromagnetic material.
This imparts greater sturdiness to the bottom 2B as compared with the clamping surface 2A, because the monolithic ferromagnetic material is more resistant than the assembly of different materials that forms the clamping surface 2A.
Therefore, the bottom 2B of the housing 2 has a higher strength than the clamping surface 2A.
In order to partially compensate for the poorer strength of the clamping surface 2A, the housing 2 of the magnetic apparatus 1 must be strengthened, e.g. by forming ribs and/or increasing the thickness of the bottom 2B.
The problems of poorer mechanical strength and stability of the clamping surface 2A cannot be eliminated even when, for example, ribs are formed and/or the thickness of the bottom 2B is increased, because such measures have no effect on the poor stability conditions of the pole piece collector.
However, the provision of ribs reduces the magnetically active area of the clamping surface 2A, and a greater thickness of the bottom 2B increases the amount of material required for forming the magnetic apparatus 1, wherefore the useful space available to the machine tool for machining the workpiece to be machined is reduced.
It should be further noted that the stresses generated during machining can cause fractures in the filling material 100 with time.
This may cause infiltrations of cooling liquids, which may reach the electric coils 3 and generate short-circuits.
Furthermore, considering that each pole piece collector 5 is only retained by one screw 5 received in an appropriate hole 7, so that the solenoid 3-reversible magnet 4 assembly can be assembled into a pack, the pole piece collector 5 has a poor ability to withstand mechanical stresses generated during machining of ferrous workpieces.
The above clearly shows that, both for the manufacturers of magnetic apparatus for clamping ferrous elements, and for the users thereof, the need is strongly felt for quicker, simpler and more reliable assembly processes as well as for more resistant and reliable magnetic apparatus.